2. The 5 Second Rule

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You're about to listen to a brand new episode of Curious Cases.

Shows are going to be released weekly, wherever you get your podcasts.

But if you're in the UK, you can listen to the latest episodes first on BBC Sounds.

I'm Hannah Fry.

And I'm Dara O'Brien.

And this is Curious Cases.

The show where we take your quirkiest questions, your crunchiest conundrums.

And then we solve them.

With the power of science.

I mean, do we always solve them?

I mean, the hit rate's pretty low.

But it is with science.

It is with science.

Dara O'Brien.

Hannah Fry.

Where do you stand on eating food off floors?

I'll answer that.

But can we just dial the back slightly?

Sure.

Because it's fun to do this series.

But I am new here and there's been no meeting.

Like, who are these people who are sending in the questions?

format, Dara.

Yeah, but you're going to sit me down and go, Curios.

What are Curios?

This is the name that the listeners gave themselves.

The other alternatives included quidunks.

Okay.

Which I, at the time when that was suggested, looked it up and then immediately forgot what the definition was.

Right, okay.

So, curious are the listeners, right?

I'm writing this down.

Where do the questions go to?

So, they tend to come into curiouscases at bbc.co.uk.

Brilliant.

And then there's sort of questions that, you know, when you've got an annoying eight-year-old and they ask you stuff and you're not sure what the answers are.

Ah, okay.

Right.

That's our favourites.

Yeah, okay, that no, I can get behind it.

See, this is the kind of stuff that I just took the gig.

And I didn't question it too closely.

Didn't even get a welcome, welcome email.

There's no, are you, where's Fruit Basket?

Where were the trumpets?

Some sense of like, well, it's your first day.

No, you just came straight in with a question.

I think, if anything, Dara, this is a great compliment to you that you've slotted in perfectly with the team and you required no introduction.

Can you remember a time in your life when you weren't sat in this studio with me?

I can't actually at this stage.

There's a question in itself.

Like, what was BC in this situation to my AD?

BCC.

One curious cases.

Okay.

So what's the question this week?

This one was sent in by Quinn.

We even asked to settle an argument about the five-second rule.

Is it okay to eat something if you pick it up off the floor within five seconds of touching it?

I would call it the three-second rule.

Would you?

Yeah, I would.

Yeah, I would.

More bacteria in Ireland.

No, but quicker bacteria in Ireland.

Bacteria in Ireland is trained to just move fast.

Do you know what I always imagine when I think of the three-second rule?

Firstly, I always imagine toast.

Like, there's a bacteria sitting on a floor, right?

And then this toast appears and then it comes down slowly onto it.

Like the movie Inception, that the whole landscape just flips open.

Suddenly, the piece of bacteria has gone from sitting on top of a surface to being both on top of a surface and beneath another surface.

And like it's like I kind of just lasso, you know, or hook itself onto.

Is that what it's spending the first four seconds doing?

Just leaping the lasso around

trying to hook its way onto this new side.

Only on the fifth second does it.

and that's why it works as a three second rule but not as a five second rule because it's four seconds to build a grappling hook for a bacteria to hook onto the piece of toast does butter change this scenario or uh just make it a lot more slimy oh i think it would wouldn't it i mean like butter is like a it's a it's a it's an access route you know what's your general rules here actually where are your boundaries if it's soup no

That's where I draw the line.

What if it was soup on your own kitchen worktop and you were really hungry?

Do I have a straw?

No, but you have a finger.

Oh no, that's fine.

That's absolutely fine.

Yeah, yeah, yeah.

A taste thing on a workstoop is yeah.

This rule of never soup has fallen very quickly.

Well there's a volume element as well.

I mean, because I'm just, if I'm dabbing the top of the soup rather than scooping it all, I presume like I was taking my hand and I was pushing it, I was pushing it off the edge into a bowl I'm holding at the side of the bowl.

And that would be a no, huh?

No, I don't think it would be entirely a no.

Different situation.

Child with an ice cream, icing falls out.

Ah, see, now this is an extra dimension, which is the pressure of the scenario.

Yes.

Yeah.

Yeah, yeah.

I think always okay.

As long as it's not like dusty and so you can see it.

I mean, even then, I think if you can just pick the bits off.

Yeah, yeah, true.

The risk of them actually dying is very small.

Got to presume that, you know, modern medicine.

Or maybe that's, is that exactly what we have to find out?

One thing I would say that maybe there's a fourth dimension to this, because I think that there is the judgment of others.

Because if a child drops a lollipop, for example, even on a subway, I would happily just, just, you know, give it a bit of a polish back in the mouth.

Unless there was another parent watching.

Absent of all of that, would you more willingly give it to the child than you would to yourself?

Oh, yeah.

Yeah, okay, fine, yeah.

Okay, I agree with that.

So the least hygeic situation is a child where no one else can judge you and you want peace and quiet.

And then it literally doesn't matter what it is or what it's falling on.

Back in the mouth.

I think so.

Okay, well, should we find out the actual size?

I think we're obliged to.

I think it's great.

I think we've probably what little I've learned about the show in the weeks that I've been doing it is that at some point smarter people come on.

Hopefully our guest today can help us settle this matter.

Professor Sally Bloomfield is the chair of the International Scientific Forum on Home Hygiene and Dr.

Ellen Evans researches food safety behaviour at Cardiff Metropolitan University.

And Professor Don Shafner is a food microbiologist who has actually put the five-second rule to the test.

Don, okay, I think most people have heard of some version of this rule.

Do we know where it came from?

I've heard that Genghis Khan had something called the five-hour rule, which was after his horde invaded, his men were allowed to eat anything that had been on the ground for less than five hours, but I'm pretty sure that's apocryphal.

How were the stopwatches in those days?

Well, exactly.

That's why it had to be five hours.

We probably used a sundial.

But there is this idea, though, that food is sort of, I don't know, like inherently safe if you pick it off the floor after a particular period of time.

Or I guess the opposite of that is that if you leave it long enough, it becomes unsafe.

We showed that that's just really not true.

There was no

one period of time that was super short where no bacteria transferred.

So in the experiments that we did, even when we tried to pick the food up off the surface as quickly as possible, some of the bacteria transferred.

Even if it came off the floor like one second later.

Well, we didn't actually use a floor.

We actually used little pieces of floor in a laboratory.

But yes, we had an experiment where we put it down and picked it up as quickly as possible, and we always saw some bacteria transfer.

Does that mean the five-second rule is there's no particular threshold there?

See, it's complicated because for some foods and some surfaces, the longer it sat there, the more bacteria transferred.

But what I'm saying is there was no quote-unquote safe amount of time where no bacteria transferred.

So do some foods attract more bacteria from surfaces than others?

How's soup in this spectrum?

We didn't study soup, but we did study watermelon.

And I can tell you, and other people have done similar experiments to show it's the moisture of the food that really drives the transfer, right?

The bacteria are tiny, they don't have legs, some of them have flagella and they swim around, but they don't swim very fast.

So, if you drop a wet food like watermelon, you're going to transfer almost all the bacteria from that surface instantaneously.

But if you drop a dry food like a piece of bread or a gummy candy, those were two that we studied, we didn't see as many bacteria transfer.

What kind of bacteria are we talking about here though?

Is it sort of, are we talking about dangerous stuff or is it just like, you know, happy little guys who won't really cause that much harm at all?

The particular microorganism that we used was one that was not pathogenic, but there's other people have done research with pathogens and they basically behave the same way.

What about the different surfaces then?

Because if that's the food, but the surface as well, presumably this matters.

Well, it does, but in kind of a weird way.

So one of the surfaces that we studied was carpet, which was a very, you know, shallow pile carpet.

We saw less bacteria transfer from the carpet, but I think that's really more an artifact of the way that we did the experiment.

So for example, imagine we took a tiny little test tube full of liquid with bacteria in it.

We put that onto a hard surface and we put that onto a carpet surface.

Well, what happens when the bacteria went onto the carpet surface, they sunk down below the top level of the carpet.

And so there were probably fewer bacteria available at the top of that carpet to transfer.

So we saw less transfer.

That doesn't mean that we should all immediately run out and start carpeting our kitchens.

That would be a terrible idea because, of course, the dirt and the bacteria and everything is going to hide down in the carpet.

There's also a trampling factor that sounds like you haven't taken into account.

No, we did not study trampling.

So, okay, carpet bad, soup bad, watermelon bad.

Okay, often that feels quite intuitive.

John, can I ask about how bacteria transfers?

Because we have tend to maybe anthropomorphize this as if a piece of toast falls the bacteria spots the goes hello toast that's better than what I'm on at the moment and then travels towards it or wishes to go onto it as if the five-second removal was somehow the commuting limit well some bacteria have flagella which are like little tails that propel them right and some don't but I think really what's fundamentally driving this is the moisture.

And so if there's moisture there, the bacteria are attracted to the moisture and they just move with the moisture.

So they get into the moisture of the watermelon and then they move with the watermelon.

Oh, are they not like tasting sugar?

Yes, I presumed it was.

You accidentally dip it into their world and they're like, hello, I'm having a bit of this.

Broccoli, no, thanks.

Yeah.

Certainly they are attracted to things like sugar and they'll move to environments that have a higher sugar concentration or a higher nutrient concentration.

But the problem is that's happening on a completely different time scale.

Remember, these microorganisms are very small.

Even if they're swimming at full speed, they don't swim that fast.

And so really, at the point where we're studying food being dropped on the floor, it's really not about the biology.

Well, okay, I mean, because we're eliminating ants, presumably, from this inquiry, we're basically talking bacteria.

It's the only case of if you drop an ice lolly in the garden, you will attract a lot more than just bacteria.

Why stop at ants?

Let's go, mice, let's go, rats.

Absolutely, okay, granted.

All vermin.

We're including this.

Yeah, because what were the time scales you did, Don, in your experiment?

We studied a fraction of a second, five seconds, 30 seconds, and 300 seconds.

So very, very short on the time scale, probably not enough to attract ants or other vermin.

If you're waiting 300 seconds.

I mean, was that to have a base level?

Was that kind of the...

Because no one is going, I dropped some toast.

I'll come back in five minutes.

Well, microbiologists like me like to think in orders of magnitude.

So if we were going to study 30 seconds, one order of magnitude higher would be 300 seconds.

But you're right.

A person is probably not waiting 300 seconds.

Sally, can I bring you in here?

Is this the wrong way to think about hygiene?

It is slightly, yes.

Yes.

Because one thing you haven't said is most pathogenic bacteria and all viruses cannot live outside people.

So if they get into the environment, they're looking for a new host.

They need to get into your body before they can start breeding again.

So finding a bit of toast, they're not going to be able to live there.

Non-pathogenic mortgages, yes, they can live anywhere.

A lot of them are what we call ubiquitous.

But the really harmful ones need to get into a human body where they've got special nutrients like vitamins and things like that.

And the same with viruses have got to live inside, go inside living cells.

They can, you know, jig around, but until they find a new host that suits them, they won't start breathing anymore.

So it's not that they won't necessarily be on kitchen surfaces or floors.

They will, because

we move them around.

We're the culprits.

You know, most harmful microbes come from people or our our pets or our food.

And they'd stay there until we do something.

So when we cut up a chicken, we move it.

When we cough and sneeze, we move it.

So we propel microbes around our home, around our environment, and we have the power to stop them.

That's what hygiene's about.

So it's more a danger that we drop this...

By the way, the toast is my unit of currency in this entire discussion or whatever, which is the whole butterside down, butterside up thing, which is another episode, I presume.

But our piece of toast, it's more of a danger that we drop it onto a work surface we haven't cleaned after you putting a chicken down before you roast it or whatever the that yeah yeah because if you think of the way we live these days our house floors are relatively clean we keep poo away when we take the dog out for a walk we don't have horses on the streets and and this type of thing so the chances of there have been some real nasties on the floor is relatively low but that doesn't mean to say that touching things isn't important because things touch surfaces which are higher up and which we're all touching, that's where the real problem is.

Does that mean a work surface just on average is more likely to be dangerous than the floor?

Well, yes, but it's what that work surface is being used for.

You know, if it's tucked in a corner, it doesn't come into contact with harmful microbes because it's about, we call it risk management.

Microbes, pathogens are a hazard, but they're only a risk if you're exposed to them.

Okay, if you're saying that the stuff that can really harm you or hurt you comes from other animals or humans, if you had a really grimy concrete floor that happened to be in a place where there weren't any other humans around, you just eat merrily off it.

It should be all right.

But remember, remember, you've got things like chemicals and all this sort of thing that, you know, it's not just about microbes.

If you find yourself in a forest without a plate, don't worry.

Well, I would think so, unless there could be animals around, you never quite know.

But no, it's, yes, I'll stick by it.

By the way, we're really just looking for some way for this to be okay to eat off the floor.

So if you initiate a kitchen system, which it's just you, obviously, in this house, you've no pets, and you hose your kitchen down into a sluice or a drain at one end regularly, in that situation, it's okay to pick food off the floor.

Well, not even that, because how they get there in the first place.

Yeah.

They've either got to come in on your shoes or they've got to come in on the air.

And the main ones are from people, pets.

and raw foods.

Okay.

And kids eating from raisins, for example, and dropping them, picking them up, dropping them, picking them up.

That's...

I don't, yeah.

It's about

what they're doing with their hands.

Did they wash their hands before they started picking them up?

Were they on a tubed station where they touched all the rails and somebody else had touched those rails?

I'd be far more worried about their hands than I would about the floor that they were picking it up off.

I feel like this is becoming almost like a Zen exercise, that we have to...

We're focusing on the grape and you are focusing on the entire universe.

And we have got this wrong.

We have to think beyond the raisin that has been dropped and into the whole world that they're in.

Eat the raisins off the floor.

Just don't eat them with your hands.

Okay.

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There's a phrase I think you've used, the journey of the germ.

Yes.

What does that mean?

Germs are moving all the time.

I think when we had hand washing during COVID, they kept saying, wash your hands frequently.

What does that mean?

You know, just wash them frequently.

We should be washing them at a moment when there may be a risk, when we've just come home from work and we've been touching all sorts of things, when we've just been preparing a chicken and it could be contaminated.

So it's about visualising rather than just saying, oh, I was told that I must wash my hands after going to the toilet,

before handling food, after handling raw food.

We've got these sort of rules in our mind, but they don't relate to anything.

They're just suspended in space.

In fact, what they're doing is to stop germs moving from a source to us.

Okay, well, so far we've been talking quite a lot about the kitchen because of course, as you say, Sally, this is where we're doing a lot of our food preparation and and eating.

But Ellen, I know that you actually study people in their kitchens and watch what they're what they're doing when they're preparing their food.

Tell us a a little bit about that.

Anna, you make me sound as if I'm peeking in through people's windows, which is not what they do.

Just don't invite you to a dinner party.

That's the rule.

I'm definitely coming around to have dinner with you, as long as I don't have to eat off the floor.

At the university, we've got a kitchen, which is much like a kitchen you'd have at home.

But we've got some CCTV cameras in there so we can see what people do.

So it might be that they are trying to wash their hands

every time they handle raw chicken, but it's only through watching them that we we can see, you know, are they using soap?

Quite often see hands being held underneath a running tap, and then hands dried on clothing.

So, there's there's always much room for improvement in terms of

hygiene practices.

Alan, are you seriously telling me that I can't rub the bacteria off onto a cloth?

Is that I mean, I've been this seems to contradict what Don was saying earlier.

Yeah, I thought the whole point was that I could break the bacteria by rubbing my hands together and then just deflect it onto a piece of cloth instead.

That's not the case?

Well, do you need a cloth that's covered in bacteria?

You're going to hopefully put that chicken in the oven and once you've reached a safe temperature that bacteria should be killed so it's not going to make you ill.

So you're just creating problems and more work for yourself by trying to clean it.

What about other parts of cooking though?

What about like in their fridges?

Are people nice and hygienic in their fridges, Ellen?

Don't get me going about fridges.

I'm obsessed with fridge temperatures.

So the temperature of the fridge can have a huge impact on how bacteria in that food can grow.

Unfortunately, a lot of fridges don't have thermometers in them, or people don't have thermometers to use.

So, people just trust that the fridge is cold enough.

You open a fridge, you take the milk out.

If it's at 7.5 degrees, it's still going to feel cold.

It's quite difficult for us humans to tell just by touch: is this less than five degrees?

Is it just above five degrees?

And when it comes to something like listeria, if we've got something at 7.5 degrees Celsius, it's going to grow much, much quicker than at five degrees can i ask a question about the reheating of things is it chicken is the great danger in terms of reheating what where what is i thought it was rice rice danger to reheat

yes rice contains spores organisms that resist drying so out in the fields these bacillus organisms which are what we're talking about get onto the rice and it's it's resistant to to heat but what the heat does is to cause the spores to germinate and so if you eat the rice immediately after it's been boiled and eat it, it's perfectly safe.

But then you leave it at room temperature, those spores have germinated and can start growing.

And so therefore, if you don't finish your takeaway, let's say, or you're eating meal, the rice has to go.

It really should, yeah.

Or just eat it in one.

That's fine.

That's also.

That's okay as a second planet.

That also works.

But in the unlikely situation that it's left over, that's the bad thing to leave in the period.

You know, you may reheat the food and get rid of the bacteria, but the toxins that they've produced is still in the food.

So that's another reason for not reheating things.

It depends on the organism, but

you're anti-reheating at all.

I mean, if I keep it in the fridge, then there's quite a lot of things I do reheat.

So it's not a blanket thing, but something you've got to be careful of.

What about reheating twice?

Oh, dear.

You would, wouldn't you, ask that?

I've no idea.

Oh, fine.

Sorry.

I presume they're saying that you're going, oh, Hannah, it's a wonder you've survived this far.

That's an interesting woman.

Let me offer maybe a slightly different perspective.

I had leftover rice for breakfast this morning.

I will have leftover rice probably for a couple of more days.

I'm not worried because I know the temperature of my fridge.

But most importantly, and I don't think either Ellen or Sally have touched on this, but I think they'll agree it's quite important.

If you're going to cool a cooked food, you need to cool that food promptly.

That's where the risk from Bacillus cereus in rice comes in.

Cool the food quickly and promptly.

If there are no pathogenic bacteria in the food once it's cooled and in the fridge, they're not going to be spontaneously generated.

And I will reuse rice multiple times.

But again, I know my fridge temperature and I know that I properly cooled that rice before I put it into the fridge.

And same with chicken and other foods.

Honestly, I've always let things cool gently before I put them into the fridge.

I've never thought that you instantly, once you've decided, I'm not eating this, you put the hot food into the fridge.

You could let it cool at room temperature for a bit, but after an hour, then I would definitely move it to the fridge.

One One thing you don't want to do, Dara, is put hot food in the fridge because then you can be increasing the temperature of the hydrogen.

Of the fridge.

And it can take ages for that temperature to come back down to normal.

It's just a minefield everywhere you can.

It would be difficult to do this.

It's no wonder that we are still alive.

The risks of not eating still outweigh the risks of eating.

So we have to keep eating, folks.

Okay, that's good.

Well, I mean, if there's any message to take from this, that is a reassuring one.

Sally, clearly we need and try to be clever about home hygiene.

But there was an idea in the first round that it's good for us to expose ourselves to different germs, that we can be too clean.

Is this ridiculous modern notion, or is this true?

Oh, the hygiene hypothesis, the threaded hygiene hypothesis.

I mean, underneath there is a real truth here, but because it's become called the hygiene hypothesis, it's become distorted out of all imagination to be we need exposure to harmful pathogens, germs to keep our immune system strong.

And that is rubbish.

Yes, we do need exposure to microbes, but it's about the the right sort of microbes.

The point is that we don't need a strong immune system, we need a well-regulated immune system.

That is, we need an immune system that, when it meets a pathogen, a harmful organism, it reacts and will destroy those organisms.

But when our body sees things like pollen and foods,

if it does react to those and it can react to those, that's when we get allergies and that's when we get problems.

So, what we need is our immune system to be regulated so that it kills harmful microbes and tolerates other things.

How do we get to that immune system?

Well it's because of exposure to microbes, these old friends microbes, okay?

And the best way to describe it is to think of our immune system like a computer program.

We're born with a fully formed immune system, but as we go through the first days, months, hours and so forth of our life, we are gradually exposed to more and more organisms.

When we come down the birth canal, we're exposed to these important old friends organisms.

When our mother breastfeeds us, we're exposed to them.

With our sisters and brothers, we're exposed to them.

Importantly, also, when we go outdoors, so it's not just human organisms, it's also outdoor organisms.

And these old friends program our immune systems so they will react to harmful things and they won't react to non-harmful things.

So, if you think about it, it's actually not about hygiene, it's about lifestyle.

It's finding ways of making sure that children do interact with their environment, but hygiene is applied, and we were talking about targeted hygiene.

Again, that's why targeted hygiene is so important, because it maximises protecting us against harmful organisms and leaves us free to be exposed to other types of important organisms.

So allowing children to play fairly freely

outdoors.

Yes, I mean, look, they'll sit in a garden or a park, whatever, and they will put things in their mouths or whatever.

That sort of thing is a fairly natural process.

Yes, it is.

And I mean, sometimes it happens, you know, that people do get ill, but they don't happen that often, and we have to live with risk.

There's a nice idea then,

because I think that people sort of have taken this, like, you're living in too sterile an environment to mean that you should be exposing yourself sort of willy-nilly, as it were, just, you know, all dirt is good dirt.

But you're saying that it's very specific types of microbes that are important.

Yes, and we call them old friends.

It's my colleague, Professor Graham Rook, who came up with this idea of old friends.

And he's saying that it was when we were back in the hunter-gatherer times, when we were living within our environment and our immune system was developing.

And it was at that moment that these organisms took over this role of programming the immune system.

Now, don't you, I can see, Diary, you're giving me one of those looks.

What?

Look.

Because

even Graham Rook can't explain to me exactly how it happens biochemically.

Because if we could find out, is it an organism or is it a particular molecule in this organism?

Because if it was, then we could start exploiting this for treatments and for preventive treatments.

So we're not there yet, but there's very, very strong evidence for the old friends hypothesis.

But we are getting better at recognizing ourselves as this term, the term microbiome, which is only, which is very, very new, at recognising ourselves as working in tandem

with these bacteria.

You know, we're full of microbes and they are so important to us in the way of regulating all sorts of things, not just immunity.

We do have to consider those clinically vulnerable populations in society that have a weak immune response to invading pathogens.

So, children at the age of five, the immune system is still developing.

And then, as we get older, the immune system doesn't work quite as efficiently as when it was younger.

We can have certain illnesses which can have an impact on immune response, or also medications, so chemotherapy treatment, for example, that that can have an impact on the immune system or or taking certain types of drugs honest there's so many factors to be taken into account i i'm going to cut through to very because there's three of you and i want a very simple vote on this you're on your own you're in your kitchen you're hungry you're very hungry actually let's make it you're very hungry okay you've won piece of bread because you didn't buy new bread you've buttered the piece of bread it falls onto the floor no one is there no one will judge you but you have walked on the floor in your shoes yes you have you have it's a normal floor that you've been in in.

And the piece has fallen, like whatever.

Just, I'll start with you, Ellen.

It's simple yes or no.

Would you eat the piece of toast?

Oh, has it landed butterside down?

60% of the time it does.

You're going to have to take the altar if it is, yeah.

I think if it was better side up, I probably would.

Okay, but only butterside up.

Don, are you eating the piece of toast?

Butterside up, butterside down.

I'm going to live a life on the edge.

Okay.

And finally, Sally, are you eating the piece of toast?

Yeah, because I think you have to learn to live with risk.

And for me, that's such a small risk.

I'd go for it.

Well, you're all filthy, a lot of you.

I wouldn't go near myself.

I was like, I don't know how you can live like this in a style.

Just this guy.

I'm so delighted.

We had three hygiene researchers all who went, how hungry am I?

Yeah,

I need to toast.

I'd go for it.

And here's the thing:

if the bread fell in a pile of my dog's poo, I would not eat it.

Okay, all right.

Let's not change it.

I mean, I would consider

that bizarre hypothetical scratch.

Just for the floor.

Good to know where your limit is.

Yes.

And good to know you have it.

You were going to ask us about eating soup off the carpet, Dara.

We'll take that as a line in the sand.

Thank you to all of our guests, Professor Don Schaffner, Professor Sally Bloomfield, and Dr.

Ellen Evans.

Oh, there you go, Quinn.

I think we've answered your question.

Yeah, eat what you want.

There are no rules.

It just seemed, I mean, it turns out the five-second rule was, if anything, too strict.

Just don't lick a chicken, and that's about it.

Don't keep rice in the fridge for too long.

But, God, yeah, whatever's in the ground, go for it.

And hygiene researchers are absolute filthmongers.

Honestly, I mean, that must be what happens, though.

You research it and you're kind of going, why have we been worried about this?

Turns out it's all fine.

Oh, I poured soup in the floor.

It doesn't matter.

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